Semiconductor device with asymmetrical pinned magnets, and method of manufacture

ABSTRACT

A device is provided that includes a semiconductor substrate on which a free magnetic element is positioned, which has first and second magnetic domains separated by a domain wall. A first magnet is positioned on the substrate near a first end of the free magnetic element, and has a first polarity and a first value of coercivity. A second magnet is positioned on the substrate near a second end of the free magnetic element, and has a second polarity, antiparallel relative to the first polarity, and a second value of coercivity different from the first value of coercivity.

BACKGROUND

Spintronic memory devices have been proposed as components in artificialneural network architectures.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present disclosure are best understood from the followingdetailed description when read with the accompanying figures. It isnoted that, in accordance with the standard practice in the industry,various features are not drawn to scale. In fact, the dimensions of thevarious features may be arbitrarily increased or reduced for clarity ofdiscussion.

FIG. 1A is a side elevation diagram of a magnetic tunnel junction (MTJ).

FIG. 1B is an equivalent electrical schematic diagram of the MTJ of FIG.1A.

FIG. 2A is a side elevation diagram showing a spintronic memory device.

FIG. 2B is an equivalent electrical schematic diagram of the spintronicmemory device of FIG. 2A.

FIG. 3 is a graph showing magnetization vs. magnetic field strength offirst and second magnetic samples in an oscillating magnetic field.

FIG. 4 is a side elevation diagram of a spintronic memory device,according to an embodiment.

FIG. 5 is a simple flow chart outlining a method for magnetizing firstand second ferromagnets, according to an embodiment.

FIG. 6 is a graph showing the magnetic responses of the first and secondferromagnets during the steps of the process of FIG. 5.

FIG. 7 is a chart showing values of coercivity plotted against diametersof test samples.

FIG. 8 is a side elevational diagram of a spintronic memory device,according to another embodiment.

FIG. 9 is a diagrammatic top plan view of the memory device of FIG. 8,showing shapes and arrangement of elements of the memory device,according to one embodiment.

FIG. 10 is a diagrammatic top plan view of the memory device of FIG. 8,according to an alternate embodiment.

FIGS. 11-13 are side elevational diagrams of a semiconductor device thatincludes a spintronic memory device similar to the memory device of FIG.8, at respective stages of a manufacturing process, according to anembodiment.

FIG. 14 is a flow chart outlining a method of manufacture, according toan embodiment.

FIG. 15 is a side elevational diagram of a semiconductor device,according to an embodiment, that includes a spintronic memory devicesimilar to the memory device of FIG. 8, at a stage of a manufacturingprocess that corresponds to the stage shown in FIG. 13.

FIG. 16 is a side elevational diagram of a semiconductor device,according to an embodiment, that includes a spintronic memory devicesimilar to the memory device of FIG. 8, at a stage of a manufacturingprocess that corresponds to the stage shown in FIG. 13.

FIG. 17 is a top plan view of the semiconductor device of FIG. 16 at thesame stage of manufacture, according to an embodiment.

DETAILED DESCRIPTION

The following disclosure provides many different embodiments, orexamples, for implementing different features of the provided subjectmatter. Specific examples of components and arrangements are describedbelow to simplify the present disclosure. These are, of course, merelyexamples and are not intended to be limiting. For example, the formationof a first feature over or on a second feature in the description thatfollows may include embodiments in which the first and second featuresare formed in direct contact, and may also include embodiments in whichadditional features may be formed between the first and second features,such that the first and second features may not be in direct contact. Inaddition, the present disclosure may repeat reference numerals and/orletters in the various examples. This repetition is for the purpose ofsimplicity and clarity and does not in itself dictate a relationshipbetween the various embodiments and/or configurations discussed.

In the drawings, hollow arrows are used to indicate magnetic or electronspin polarity within a structure or region, relative to polarities ofother structures or regions in a same drawing. These arrows are notintended to indicate any specific polarity, nor even a specific axis ofpolarity. Where regions, structures or domains are referred to as beingparallel, this indicates that polarities of the elements referred to areoriented substantially in a same direction and parallel to a same axis.The term antiparallel refers to polarities that are not oriented in asame direction or parallel to a same axis. While antiparallel typicallysuggests that one polarity is oriented at 180° with reference to anotherpolarity, there may be instances where the angular difference oforientation is other than 180°.

Further, spatially relative terms, such as “beneath,” “below,” “lower,”“above,” “upper” and the like, may be used herein for ease ofdescription to describe one element or feature's relationship to anotherelement(s) or feature(s) as illustrated in the figures. The spatiallyrelative terms are intended to encompass different orientations of thedevice in use or operation in addition to the orientation depicted inthe figures. The apparatus may be otherwise oriented (rotated 90 degreesor at other orientations) and the spatially relative descriptors usedherein may likewise be interpreted accordingly.

As used in this specification and the appended claims, the singularforms “a,” “an,” and “the” include plural referents unless the contentclearly dictates otherwise. It should also be noted that the term “or”is generally employed in its sense including “and/or” unless the contentclearly dictates otherwise.

FIG. 1A is a side elevation diagram of a magnetic tunnel junction (MTJ)100, while FIG. 1B is an equivalent electrical schematic diagram of theMTJ. The MTJ 100 is formed by first and second ferromagnets 102, 104that are separated by a tunneling barrier 106. When an electricpotential is applied across the MTJ 100 at terminals 108 and 110, theMTJ behaves as an electrical resistor R₁ having a resistive value thatdepends upon the relative polarities of the first and secondferromagnets 102, 104. If, for example, the first and secondferromagnets 102, 104 are oriented with their respective polarities inopposition—i.e., antiparallel—as shown in FIG. 1A, the resistive valueof the resistor R₁ will be relatively high. On the other hand, if therespective polarities have the same orientation, i.e., are parallel, theresistive value of the resistor R₁ will be relatively low. Thisdifference in resistance is due to tunnel magneto resistance (TMR). Theactual ohmic values of the relative high and low resistances depend uponseveral factors, including the compositions of the first and secondferromagnets 102, 104 and of the tunneling barrier 106, the dimensionsof the first and second ferromagnets, and the material and thickness ofthe tunneling barrier.

FIG. 2A is a structural diagram showing a spintronic memory device 200,while FIG. 2B is an equivalent electrical schematic diagram of thespintronic device. The spintronic memory device 200 includes a writelayer 202, a ferromagnetic free layer 204, and a first pinnedferromagnet 102 separated from the free layer 204 by a tunneling barrier106. The ferromagnetic free layer 204 has oppositely polarized magneticdomains (represented by the arrows within free layer 204) separated by adomain wall 210. The magnetic domains of the free layer 204 arestabilized by the second and third pinned ferromagnets 206, 208, whichare positioned at respective ends of the free layer 204. A writeterminal 212 is electrically coupled to the write layer 202 near thesecond pinned ferromagnet 206 and a ground terminal 110 is electricallycoupled to the write layer 202 near the third pinned ferromagnet 208. Aread terminal 108 is electrically coupled to the first pinnedferromagnet 102.

Programming of the spintronic memory device 200 is performed by theapplication of an electrical potential between the write and groundterminals 212, 110. A write current generated in the write layer 202creates spin orbit torque that moves the domain wall 210 to the left orright, depending upon the polarity of the electrical potential relativeto the orientation of the magnetic domains of the ferromagnetic freelayer 204. The distance, or amount of movement of the domain wall 210depends upon the strength of the write current. When the electricalpotential is removed, the position of the domain wall 210 separating themagnetic domains of the free layer 204 is stabilized by the second andthird pinned ferromagnets 206, 208, and remains in the positionestablished by the generated current. When a read potential is appliedacross the read and ground terminals 108, 110, a read current isgenerated whose value depends upon the TMR across a MTJ formed by thefirst pinned ferromagnet 102, the tunneling barrier 106 and theferromagnetic free layer 204. The TMR, in turn, is controlled by theposition of the domain wall 210.

The memory device described above with reference to FIGS. 1A, 1B, 2A and2B has shown significant promise for use in forming synapses and neuronsin artificial neural network systems, because of its ability to hold ananalog value. However, the inventors have recognized that there arepotential problems that could interfere with production of such devices,in particular, the production of closely positioned magnetic elementshaving antiparallel polarities. A magnet is generally made by firstforming the element of an appropriate magnetic material, thenmagnetizing the element by exposure to a strong magnetic field of adesired polarity. Given the very small size of the ferromagnets of thememory device—perhaps less than a few hundred nanometers (nm) across—itmay be extremely challenging to apply a magnetic field of one polarityto a single one of the ferromagnets without affecting the polarities ofother ferromagnets nearby.

The inventors have recognized that one magnetic element can bemagnetized at one polarity without significantly affecting the polarityof an adjacent magnetic element by providing magnetic elements withdifferent values of coercivity, as explained below.

Typically, when an element of ferromagnetic material is subjected to astrong magnetic field, it will become magnetized, i.e., it will retainsome residual magnetism when the field is removed. The strength of themagnetization will vary, up to a maximum strength, or saturation, indirect relation to the strength of the magnetic field. The polarity ofthe field, relative to the element, will determine the magnetic polarityof the residual magnetism. If, after having been magnetized at a firstpolarity, the element is then subjected to a magnetic field of anopposite polarity of sufficient strength, the element will becomedemagnetized, then remagnetized at the opposite polarity.

FIG. 3 is a graph showing magnetization vs. magnetic field strengthi.e., flux density, of first and second magnetic samples in anoscillating magnetic field. The magnetization M of each sample is shownin the vertical axis, while the field strength H is shown in thehorizontal axis. The samples are initially magnetized from zero, asindicated by respective initial magnetization curves S_(1A), S_(2A).Thereafter, the plotted response characteristics of the first and secondsamples follow respective hysteresis loops S₁, S₂. The points where theloops cross the vertical axis M define the remanence M_(R1), M_(R2) ofthe respective samples, i.e., the residual magnetism of the samples whenthe magnetic field drops, from a flux density sufficient to saturate therespective sample, to zero; the points where the loops cross thehorizontal axis H define the coercivity H_(C1), H_(C2) of the respectivesamples, i.e., the field strength required to reduce the residualmagnetism of the respective sample to zero.

For the purposes of this disclosure, coercivity can also be thought ofas a measure of the ability of a given magnetic sample to retainmagnetization with a first polarity when subjected to a magnetic fieldof the opposite polarity, or in other words, the degree of resistance ofthe sample to demagnetization by an opposing magnetic field. The unitused herein for defining values of coercivity is the oersted (Oe), whichis equal to one dyne per Maxwell, in the CGS system. As previouslynoted, hollow arrows are used in the drawings to indicate relativemagnetic or electron spin polarities of elements depicted in a samedrawing. Additionally, arrow size is used to indicate relative values ofcoercivity, i.e., a large arrow indicates an element whose coercivity isgreater than that of an element with a relatively smaller arrow, etc.

FIG. 4 is a side elevation diagram of a spintronic memory device 400,according to an embodiment of the present disclosure. The spintronicmemory device 400 includes a write layer 202 on which a ferromagneticfree layer 204 is formed, including oppositely polarized magneticdomains separated by a domain wall 210. A first pinned ferromagnet 402is separated from the free layer 204 by a tunneling barrier 106. Secondand third pinned ferromagnets 404, 406 are positioned at respective endsof the free layer 204. A read terminal 108 is electrically coupled tothe first pinned ferromagnet 402, while write and ground terminals 212,110 are electrically coupled near respective ends of the write layer202. The write layer 202 is a conductive material in which spin orbittorque is generated by a write current flowing between the write andground terminals 212, 110. According to an embodiment, the write layer202 is a layer of heavy metal or heavy metal alloy, such as, e.g., Pt(platinum), Ta (tantalum), PtMn (platinum-manganese), etc.

The term pinned refers to an element whose magnetization is fixed orsubstantially fixed, at least to the extent that magnetic fieldsproduced during normal operation of the device will have no effect onits magnetization. In contrast, a free magnetic element, such as thefree layer 204, is configured such that its magnetization can bemanipulated during operation.

In the embodiment of FIG. 4, characteristics of the memory device 400are controlled during the manufacturing process such that the coercivityof the third pinned ferromagnet 406 is greater than that of the secondpinned ferromagnet 404, which in turn is greater than the coercivity ofthe first pinned ferromagnet 402. For its part, the coercivity of theferromagnetic free layer 204 is relatively much lower than that of anyof the first, second, or third pinned ferromagnets 402, 404, 406.Following manufacture of the device, the first and second pinnedferromagnets 402, 404 are magnetized according to a first polarity,while the third ferromagnet is magnetized with a second polarity,antiparallel to the first polarity. Following magnetization of thepinned ferromagnets and before beginning normal operation of the memorydevice 400, the domain wall 210 is moved, by application of a writecurrent of the appropriate strength and polarity, to near or beyond theedge of a magnetic tunnel junction region, i.e., the portion of theferromagnetic free layer 204 that is directly opposite the first pinnedferromagnet 402. This results in a TMR that is either at a minimum or amaximum value, depending upon the relative polarities of the firstferromagnet 402 and the junction region of the free layer 204. Either ofthese TMR values (minimum or maximum) can be designated as a zero, null,or unprogrammed condition of the device.

A read current is limited by the tunnel magneto resistance (TMR) of amagnetic tunnel junction (MTJ) formed by the combination of the firstpinned ferromagnet 402, the ferromagnetic free layer 204, and thetunneling barrier 106. The total resistive value of the TMR isessentially the combined values of three parallel resistances: theresistance of a first portion of the MTJ on one side of the domain wall210, in which the domains of the first pinned ferromagnet 402 and thefree layer 204 are parallel, and whose resistance per unit of area isrelatively low; the resistance of a second portion of the MTJ on theopposite side of the domain wall, in which the domains of the firstpinned ferromagnet 402 and the free layer 204 are antiparallel, andwhose resistance per unit of area is relatively high; and the resistanceof a third portion of the MTJ occupied by the domain wall itself, andwhose resistance per unit of area is between those of the first andsecond portions of the MJT. The electrical resistance of the domain wall210 remains substantially constant and, because the domain wall is onlya few molecules in width, has a minimal effect on the total TMR. Thus,control of the total resistive value is dominated by the first andsecond resistances of the respective first portion of the MJT and thesecond portion of the MJT. The sizes of the first and second portions ofthe MTJ, and thus their relative influence on the TMR, vary in inverserelation as the domain wall 210 is moved.

On its own, the ferromagnetic free layer 204 is a relatively unstablemagnet. The second and third pinned ferromagnets 404, 406 can be thoughtof as forming, with the free layer 204, one continuous magnet, withpoles defined by the second and third ferromagnets 404, 406. Thiscontinuous magnet is less susceptible to unintended changes inmagnetization, i.e., unintended movements of the domain wall 210 thanthe free layer alone. This is only possible if the second and thirdpinned ferromagnets 404, 406 are antiparallel with respect to eachother.

FIG. 5 is a flow chart outlining a method 500 for magnetizing first andsecond ferromagnets, according to an embodiment. For the purposes ofthis discussion, it can be assumed that the magnetic characteristics ofthe first and second ferromagnets are similar to those of the first andsecond ferromagnetic samples discussed above with reference to FIG. 3.FIG. 6 is a graph showing the magnetic responses of these first andsecond ferromagnets during the steps of the process 500 of FIG. 5. Forcontext, portions of the characteristic hysteresis loops of the firstand second ferromagnetic samples of FIG. 3 are also shown in FIG. 6, atS₁, and S₂.

In accordance with the embodiment of FIG. 5, first and secondferromagnets are provided, in step 502, having respective differentvalues of coercivity H_(C1), H_(C2), corresponding, in this embodiment,to the coercivities of the first and second ferromagnetic samples ofFIG. 3—structures and methods for controlling coercivity are describedin detail below. In step 504, the first and second ferromagnets FM₁, FM₂are exposed to a first magnetic field that has a flux density FD₁sufficient to saturate both ferromagnets at a first polarity, from anon-magnetized condition, as shown in FIG. 6, at FM_(1A) and FM_(2A).

In step 506, the first and second ferromagnets are exposed to a secondmagnetic field whose polarity is opposite that of the first magneticfield, and that has a flux density FD₂ that is greater than thecoercivity H_(C2) of the second ferromagnet but less than the coercivityH_(C1) of the first ferromagnet. The responses of the first and secondferromagnets to the second magnetic field are shown in FIG. 6 at FM_(1B)and FM_(2B). When the second magnetic field is removed, the first andsecond ferromagnets each retain a residual magnetism, as indicated atFM_(1C) and FM_(2C), respectively.

As shown at FM_(1C), because the flux density FD₂ of the second magneticfield does not exceed the coercivity H_(C1) of the first ferromagnet,the residual magnetization of the first ferromagnet is reduced onlyslightly from saturation in response to the second magnetic field; thefirst ferromagnet remains strongly magnetized at the first polarity.Meanwhile, the flux density FD₂ of the second magnetic field does exceedthe coercivity H_(C2) of the second ferromagnet. Consequently, as shownat FM_(2C), the second ferromagnet is remagnetized at the oppositepolarity, and retains a strong residual magnetization at that polarity.The result of this process is that even though both the first and thesecond ferromagnets are subjected to the same process, they becomemagnetized at opposite polarities.

The method described above has particular value in circumstances whereit is desirable or necessary to magnetize two or more closely positionedmagnetic elements at opposite polarities, or in quickly magnetizing alarge number of magnetic elements of different polarities. Referring,for example, to the memory device 400 of FIG. 4, it is contemplated thatsuch a device may be formed on a semiconductor material substrate aspart of a memory array, an artificial neural network processor, a moreconventional processor, etc., and so might be one of millions orbillions of similar devices, and therefore sized accordingly. Theinventors have recognized that if the magnetic elements of a memorydevice similar to that described above with reference to the documententitled Proposal for an All-Spin Artificial Neural Network: EmulatingNeural and Synaptic Functionalities Through Domain Wall Motion inFerromagnets is provided with similar or identical magnetic properties,at least one of the elements will need to be selectively exposed to amagnetic field of sufficient strength to magnetize the element whilebeing limited, focused, or constrained to a degree necessary to avoidaffecting other elements of the same device. Given the possibledimensions and quantities involved, particularly in the mass productionof commercial systems, this is likely to prove to be, at the least, achallenging, technically complex, and time-consuming process. Theinventors have conceived of a device configuration in which magneticelements having selected different coercivities are provided, whichenables application of the principles described above with reference toFIGS. 5 and 6. In accordance with these principles, it is possible toeffectively and efficiently magnetize the magnetic elements of eachdevice on a substrate.

Controlling the magnetic characteristics of a magnetic element,including its coercivity, can be performed in a number of ways. Forexample, according to an embodiment, the pinned ferromagnets of thememory device 400 of FIG. 4 can be formed of different magneticmaterials and alloys, including, in particular, various alloys thatinclude iron, nickel, and/or cobalt. A large number of differentferromagnetic alloys are known in the art, many with very widely variedmagnetic properties and are included within the scope of the presentdisclosure. Thus, according to an embodiment, alloys used in themanufacture of the memory device 400 are selected such that at least oneof the pinned ferromagnets has a coercivity that is different than thatof others of the ferromagnets.

The inventors have also recognized that it is desirable to minimize theprocesses required to manufacture semiconductor-based devices andsystems, and that therefore it would be beneficial to manufacture manyor all of the ferromagnets simultaneously, using the same materials andprocesses for each. Therefore, according to another embodiment, amanufacturing process is provided in which coercivity of theferromagnets is selected and controlled by selection of the shape, size,mass, and/or aspect ratio of each of the ferromagnets.

Experiments have been conducted in which magnetic samples made from analloy of cobalt, iron, and boron (CoFeB) were produced and tested forcoercivity. Each sample had a thickness of 1.3 nm, with diameters thatvaried from as large as around 500 nm to as small as around 25 nm. Thecoercivities of the test samples were found to vary from less than 200Oe, in the samples with the largest diameters and lowest aspect ratios,to more than 3500 Oe, in the samples with the smallest diameters andhighest aspect ratios. According to an embodiment, the coercivities ofthe ferromagnets of the memory device 400 are selected to be greaterthan about 500 Oe, to reduce the likelihood that the magnetization ofone of the pinned elements might be unintentionally affected by anexternally generated magnetic field, and less than about 3000 Oe, toenable initial magnetization of the pinned elements without undueexpense or time. FIG. 7 is a chart showing the values of coercivityplotted against the diameters of the samples. Aspect ratios(thickness/diameter) of selected samples are also shown in the chart. Asshown in FIG. 7, CoFeB ferromagnets can be made with aspect ratios ofbetween 0.0087 and 0.037 to provide coercivities of between 500 Oe and3000 Oe.

FIG. 8 is a side elevational diagram of a spintronic memory device 520,according to an embodiment. The memory device 520 is similar instructure and operation to the device 400 described above with referenceto FIG. 4, and includes a write layer 202, a ferromagnetic free layer528, a tunneling barrier 106, and first, second, and third pinnedferromagnets 522, 524, 526. Likewise, read write and ground terminals108, 212, 110 are also provided. However, there are some structuraldifferences between the devices 400 and 520. For example, it can be seenthat in the memory device 520, the ferromagnetic free layer 528 extendsacross the entire device, and that a respective tunneling barrier layer530 is positioned between each of the second and third ferromagnets 524,526 and the free layer. These features are primarily for convenience inmanufacturing, as will be explained below, with referenced to FIGS.11-13. Another distinguishing feature of the memory device 520 is thatthe first, second, and third pinned ferromagnets 522, 524, 526 haverespective different aspect ratios, selected to provided respectivedifferent coercivities. The third pinned ferromagnet 526 has the highestaspect ratio and the correspondingly highest coercivity, while the firstand second pinned ferromagnets 522, 524 have lower aspect ratios andcorrespondingly lower coercivities. Thus, the first, second, and thirdpinned ferromagnets 522, 524, 526 are configured to be selectivelymagnetized at least two separate polarities, in accordance with themethod 500 described above.

According to an embodiment, the first and second pinned ferromagnets522, 524 are CoFeB magnets with aspect ratios that are between 0.0087and 0.016, and have coercivities of between 500 and 1000 Oe. The thirdferromagnet 526 is a CoFeB magnet with an aspect ratio of between 0.021and 0.037 Oe, and a coercivity of between 2000 and 3000 Oe. Otherembodiments are contemplated in which the acceptable range ofcoercivities is different than the range defined above, either out ofnecessity or convenience. This may be the case, for example, inaccordance with specific design criteria, intended operating conditions,surrounding structure, etc.

According to one embodiment, the ferromagnetic free layer 528 is a CoFeBmagnet, and the tunneling barrier layer 106 is of MgO (magnesium oxide),which has been found to provide a satisfactory barrier layer in a MTJ incombination with CoFeB ferromagnets. The present disclosure is notlimited to CoFeB ferromagnetic free layer or an MgO tunneling barrierlayer. The specific materials used is a matter of system design, andthere are other combinations of materials that will performsatisfactorily as the ferromagnetic free layer and the tunneling barrierlayer in appropriate conditions. Such other combinations of suitablematerials are within the scope of the present disclosure.

FIG. 9 is a diagrammatic top plan view of the memory device 520 of FIG.8, showing the shapes and arrangement of the first, second, and thirdpinned ferromagnets 522, 524, 526, and the ferromagnetic free layer 528,according to one embodiment. FIG. 10 is a diagrammatic top plan view ofthe memory device 520 of FIG. 8, according to an alternate embodiment,in which the first, second, and third pinned ferromagnets 522, 524, 526are rectangular in plan view, rather than circular. In some systems, itmay be preferable for elements of the memory device to be rectangular,or some other shape, in order, for example, to be positioned morecompactly in arrays, or to accommodate other design concerns.Embodiments that include these and other variations in shape andposition of the elements of the device are therefore contemplated.

FIGS. 11-13 are side elevational diagrams of a semiconductor device 540that includes a spintronic memory device similar to the memory device520 described above, at respective stages of a manufacturing process,according to an embodiment. The semiconductor device includes asemiconductor material substrate 542 with active and passive electronicelements formed in a semiconductor base layer as well as additionalsemiconductor material, insulation material, and interconnectionsdeposited thereon. Except for first, second, and third vias 543, 544,545 that extend from metal interconnection layers 546 to an intermediatesurface of the semiconductor material substrate 542, these and otherelements formed in the semiconductor substrate 542 are not shown indetail here, inasmuch as they are made in accordance with knownprocessing methods, and are not directly pertinent to the embodimentsdisclosed. Nevertheless, it will be understood that the wires of theinterconnection layers 546 are electrically coupled to circuits formedin the substrate as required for proper operation of the semiconductordevice 540.

Referring first to FIG. 11, following the formation in the semiconductormaterial substrate 542 of the various elements described above, variouslayers are deposited on the substrate. A heavy metal layer 548 isdeposited on the substrate 542—in electrical contact with the first andsecond vias 543, 544, which will function as the read and groundterminals 212, 110. Then, in succession, a first layer of CoFeB magneticmaterial 550, a MgO layer 552, a second layer of CoFeB magnetic material554, a layer of synthetic antiferromagnetic material 556, a cappinglayer 558, and an electrode layer 560 are each deposited. Next, as shownin FIG. 12, a first etch is performed to expose the first layer of CoFeBmagnetic material 550, and to define, in the layers 552-560, the first,second, and third pinned ferromagnets 522, 524, 526—it will beunderstood that an etch includes the deposition and patterning of aresist layer prior to the actual etching process, as well as thesubsequent removal of the remaining portions of the resist layer afterthe etching process, which processes are well known in the art. A secondetch is then performed to expose the surface of the semiconductormaterial substrate 542, and to define thereon the write layer 202 andthe ferromagnetic free layer 528. The third via 545 is also exposed atthe surface of the semiconductor material substrate 542 by the secondetch.

According to another embodiment, the first etch is performed to define,in the entire stack of layers, the shape of the heavy metal layer andthe ferromagnetic free layer 528, after which the first, second, andthird pinned ferromagnets 522, 524, 526 are defined in a second etch. Inother embodiments, the heavy metal layer 548 is defined in a separateetch so as to extend beyond the ferromagnetic free layer 528 on one ormore sides.

In the embodiment of FIGS. 11-13, each of the first, second, and thirdpinned ferromagnets 522, 524, 526 comprises a stack that includesrespective portions of the MgO layer 552, the second layer of CoFeBmagnetic material 554, the layer of synthetic antiferromagnetic material556, the capping layer 558, and the electrode layer 560. The portion ofthe MgO layer 552 in the first pinned ferromagnet 522 acts as thetunneling barrier layer 106 between the ferromagnetic free layer 528 andthe portion of the second layer of CoFeB magnetic material 554, andwhich together form the MTJ. The synthetic antiferromagnetic material556 acts to protect and stabilize the magnetization of the underlyinglayer of CoFeB magnetic material 554, essentially acting as part of thepinning function, particularly in the case of the first pinnedferromagnet 522, which is regularly subjected to a read current.Examples of materials that are used as a synthetic antiferromagneticmaterial include, Co/Pt_(x), Co/Nix, and other materials capable ofstabilizing the magnetization of the underlying layer of magneticmaterial 554. The capping layer 558 acts as an electrically conductivepassivation layer to prevent undesirable chemical or electrochemicalinteractions between the antiferromagnetic material 556 and theelectrode layer 560, the material of which is selected to make areliable electrical coupling with a read terminal connector that will beformed in a later step. Examples of materials that are used as a cappinglayer include Ru (ruthenium), Ta (tantalum) and other electricallyconductive materials capable of preventing undesirable chemical orelectrochemical interactions between the antiferromagnetic material andthe electrode layer.

Of the stacks of layers that form the second and third pinnedferromagnets 524, 526, only the respective portions of the second layerof CoFeB magnetic material 554 are typically required, although othersof the layers may provide benefits in some embodiments. However, evenwhere some of the layers are not required, they do not generally impedethe function of the device, and by forming all three ferromagnets fromthe same layers of material, the manufacturing process is considerablystreamlined. Similarly, extending the ferromagnetic free layer 528 underthe second and third pinned ferromagnets 524, 526 does not adverselyaffect the magnetic coupling between the pinned ferromagnets and thefree layer, and eliminates the need to pattern the free layer prior toformation of the pinned ferromagnets.

Turning now to FIG. 13, a dielectric layer 562 is next deposited, andthen etched to define a first opening 564 over the first ferromagnet 522and a second opening 566 over the third via 545 at the surface of thesubstrate 542. A layer of conductive material 568, such as a metal,e.g., copper, aluminum, etc., is deposited over the dielectric layer 562and patterned to form fourth and fifth vias 570, 572 connected by aninterconnection 574, thereby electrically coupling the write terminal108 with the third via 545.

FIG. 14 is a flow chart outlining a method of manufacture 600, accordingto an embodiment. In step 602, a stack of layers is deposited on asemiconductor substrate, the stack including a heavy metal layer, afirst CoFeB layer, a MgO layer, a second CoFeB layer, anantiferromagnetic layer, a capping layer, and an electrode layer. Next,the stack is etched to expose the first CoFeB layer and define first,second, and third pinned ferromagnets, in step 604. In step 606, asecond etch is performed to expose the surface of the semiconductorsubstrate and define a free ferromagnet. Then, in step 608, a layer ofdielectric material is deposited, and, in step 610, etched, to formopenings over the first pinned ferromagnet and to expose an electricalcontact at the surface of the substrate. In step 612, a layer ofconductive material is deposited and patterned to electrically connectthe third pinned ferromagnet with the contact at the surface of thesubstrate. In step 614, a protective layer of dielectric material isdeposited over the device.

FIG. 15 is a side elevational diagram of a semiconductor device 620,according to an embodiment, that includes a spintronic memory devicesimilar to the memory device 520 described above, and at a stage of amanufacturing process that corresponds to the stage shown in FIG. 13.The manufacturing process of the semiconductor device 620 is similar tothat described with reference to FIGS. 11-13 and the process flow ofFIG. 14, with a few exceptions. First, the first etch is used to defineonly the first ferromagnet 522. Then an additional CoFeB layer 672 andan additional capping layer 674 are deposited, after which an etch isperformed to define the second and third ferromagnets 524, 526. Fromthis point, the process proceeds as described with reference to FIGS.11-14, continuing from the deposition of the dielectric layer 562, asdescribed with reference to FIG. 13.

The process of FIG. 15 includes additional process steps, compared tothe process of FIGS. 11-14. However, in the device 620, extraneouslayers of the second and third ferromagnets 524, 526 are omitted, ascompared to the device 540 of FIGS. 11-13. This may be beneficial, forexample, in embodiments in which those extraneous elements produce someundesirable effect.

FIG. 16 is a side elevational diagram of a semiconductor device 640,according to an embodiment, that includes a spintronic memory devicesimilar to the memory device 520 described above, at a stage of amanufacturing process that corresponds to the stage shown in FIG. 13.FIG. 17 is a top plan view of the semiconductor device 640 at the stageof manufacture shown in FIG. 16. The manufacturing process of thesemiconductor device 640 is again similar to that described withreference to FIGS. 11-13 and the process flow of FIG. 14, except asexplained hereafter. First, the first layer of CoFeB magnetic material550 is etched to define regions that will function as first and secondferromagnets 642, 644 and a free ferromagnetic strip 646 extendingbetween them, prior to the deposition of the remaining layers in thestack. The remaining layers are deposited and etched to define the firstpinned ferromagnet 522, and the process continues thereafter asdescribed with reference to FIGS. 11-14. Coercivities of the first andsecond ferromagnets 642, 644 are controlled by selection of the shape ofthe portion of the first layer of CoFeB magnetic material 550 thatremains after the first etch, as shown, for example, in FIG. 17. Thefirst and second ferromagnets 642, 644 are defined by enlarged regionsat each end of the free ferromagnetic strip 646, which modify theeffective aspect ratio of those portions.

It will be noted that in the various memory devices shown and describedabove (400, 520, 540, etc.), of the three pinned ferromagnets, theferromagnet in the center is shown with a coercivity that is lower thanthat of either of the others, while the ferromagnet on the right isshown having the highest coercivity. Additionally, the polarities of thevarious ferromagnets is shown in each embodiment as being the same asthe polarities of corresponding magnets of other embodiments. This isdone for simplicity and clarity, but is not intended to limit the claimsin any way. Embodiments are also contemplated that include various otherconfigurations. For example, according to an embodiment, the diameter,and therefore the aspect ratio of at least one of the pinnedferromagnets is selected to provide specific conduction characteristicsduring a read cycle of the memory device. Accordingly, the size, aspectratio, and/or coercivity of that magnet may be, out of necessity orconvenience, greater, equal to, or less than that of one or both of theother ferromagnets, depending upon the desired operating characteristicsof the device.

According to the principles disclosed, first and second pinned orotherwise fixed anchor magnets with antiparallel polarities arepositioned at or near respective ends of a free magnetic element, actingto stabilize a domain wall of the free magnet. The specific polaritiesof the anchor magnets is a design consideration. A pinned read magnet ispositioned over the free magnet and separated therefrom by a tunnelingbarrier material—typically an oxide—so as to form, with the tunnelingbarrier and the free magnet, a magnetic tunnel junction. The specificpolarity of the read magnet is also a design consideration. Preferably,the polarities of the read magnet and of one of the anchor magnets aresubstantially parallel with each other and antiparallel to the other ofthe anchor magnets.

The magnetization methods described above, particularly with referenceto FIGS. 4 and 5, can be adapted to accommodate any ordering ofcoercivities and/or desired relative polarities of the magnets. Forexample, given two anchor magnets and one read magnet, at least one ofthe anchor magnets will have a coercivity that is either higher or lowerthan both of the other magnets. The strength of the second magneticfield employed in the magnetizing process is simply selected to fallbetween the values of coercivity of that magnet and the other two.Preferably, the coercivity of one of the anchor magnets is selected tobe significantly higher or lower than the coercivities of the other twomagnets, in order to reduce the degree of demagnetization of thestronger one or two of the magnets during exposure to the secondmagnetizing field.

The embodiments described in the present disclosure illustrate severalbeneficial principles. For example, one or more ferromagnets can bemagnetized at one polarity without changing the magnetic polarity ofanother one or more antiparallel ferromagnets by controlling thecoercivities of the various magnets. This is of particular value insituations where it impractical to isolate the magnets that are to be ofone polarity from those of another polarity. Coercivity can becontrolled by selecting the shape, and in particular the aspect ratio ofthe respective ferromagnets. This is beneficial in the manufacture ofmicroelectronic devices, inasmuch as it enables formation of largequantities of magnetic structures on semiconductor substrates using wellknown and understood processes, and controlling their respectivecoercivities as a function of shape, rather than composition. Thisreduces complexity and cost. Several methods are disclosed, for themanufacture of devices that include ferromagnets whose coercivities arecontrolled by selection of their aspect ratios. Other methods aredisclosed for magnetizing antiparallel magnets.

According to an embodiment, a device is provided that includes asemiconductor substrate, a free magnetic element on the semiconductorsubstrate and having first and second magnetic domains separated by adomain wall, a first magnet positioned on the substrate near a first endof the free magnetic element and having a first polarity and a firstvalue of coercivity, and a second magnet positioned on the substratenear a second end of the free magnetic element and having a secondpolarity, antiparallel with respect to the first polarity, and a secondvalue of coercivity, different from the first value of coercivity.

According to an embodiment, a method is provided, which includesmagnetically saturating, at a first polarity, first and second magneticelements that are positioned together on a substrate by exposing thefirst and second magnetic elements to a first magnetic field that has afirst flux density, then remagnetizing the first magnetic element at asecond magnetic polarity antiparallel to the first polarity withoutchanging the polarity of the second magnetic element by exposing thefirst and second magnetic elements to a second magnetic field that has asecond flux density, less than the first flux density.

According to another embodiment, a method of manufacture is provided,that includes forming a free magnetic element on a semiconductorsubstrate, forming a first magnet, with a first value of coercivity, onthe substrate near a first end of the free magnetic element, and forminga second magnet, with a second value of coercivity different from thefirst value of coercivity, on the substrate near a second end of thefree magnetic element.

The methods described above include processes that are not described indetail, but that are common in the manufacture of semiconductor devices.For example, in deposition of a number of layers of various compositionsis described. It is understood that “depositing” some types of materialtypically involves a vapor deposition process, while for other types, asputter deposition is more common. In other cases, depositing a layermay require forming an oxide over an existing layer, or depositinganother material, then forming the oxide. In other steps, an etch isreferred to, which will be understood as including the deposition andpatterning of a resist layer, then, after the etch itself is complete,removing the remaining resist. It is easily within the abilities of aperson having ordinary skill in the art to select and perform all of theappropriate process steps that are implicit in the method steps outlinedabove.

Ordinal numbers, e.g., first, second, third, etc., are used in theclaims according to conventional claim practice, i.e., for the purposeof clearly distinguishing between claimed elements or features thereof,etc. Ordinal numbers may be—though not necessarily—assigned simply inthe order in which elements are introduced. The use of such numbers doesnot suggest any other relationship, such as, order of operation,relative position of such elements, etc. Furthermore, an ordinal numberused to refer to an element in a claim should not be assumed tocorrelate to a number used in the specification to refer to an elementof a disclosed embodiment on which that claim reads, nor to numbers usedin unrelated claims to designate similar elements or features.

The abstract of the present disclosure is provided as a brief outline ofsome of the principles of the invention according to one embodiment, butis not intended as a complete or definitive description of any singleembodiment thereof, nor should it be relied upon to define terms used inthe specification or claims. The abstract does not limit the scope ofthe claims.

The foregoing outlines features of several embodiments so that thoseskilled in the art may better understand the aspects of the presentdisclosure. Those skilled in the art should appreciate that they mayreadily use the present disclosure as a basis for designing or modifyingother processes and structures for carrying out the same purposes and/orachieving the same advantages of the embodiments introduced herein.Those skilled in the art should also realize that such equivalentconstructions do not depart from the spirit and scope of the presentdisclosure, and that they may make various changes, substitutions, andalterations herein without departing from the spirit and scope of thepresent disclosure.

The various embodiments described above can be combined to providefurther embodiments. All of the U.S. patents, U.S. patent applicationpublications, U.S. patent applications, foreign patents, foreign patentapplications and non-patent publications referred to in thisspecification and/or listed in the Application Data Sheet areincorporated herein by reference, in their entirety. Aspects of theembodiments can be modified, if necessary to employ concepts of thevarious patents, applications and publications to provide yet furtherembodiments.

These and other changes can be made to the embodiments in light of theabove-detailed description. In general, in the following claims, theterms used should not be construed to limit the claims to the specificembodiments disclosed in the specification and the claims, but should beconstrued to include all possible embodiments along with the full scopeof equivalents to which such claims are entitled. Accordingly, theclaims are not limited by the disclosure.

1. A device, comprising: a semiconductor substrate; a free magneticelement on the semiconductor substrate; a first magnet over thesubstrate and adjacent to the free magnetic element and having a firstpolarity and a first value of coercivity; and a second magnet over thesubstrate and adjacent to the free magnetic element and having a secondpolarity, antiparallel relative to the first polarity, and having asecond value of coercivity, different from the first value ofcoercivity, the first magnet and the second magnet being separated fromone another.
 2. The device of claim 1, further comprising: a thirdmagnet positioned adjacent to the free magnetic element and separatedtherefrom by a tunneling barrier layer, the third magnet, the freemagnetic element, and the tunneling barrier layer being configured suchthat, during operation of the device, an electric current passingthrough the tunneling barrier layer between the third magnet and thefree magnetic element is limited by a tunnel magneto resistance whoseohmic value is determined by a position of a domain wall within the freemagnetic element.
 3. The device of claim 2, comprising: a write elementpositioned on the substrate on a side of the free magnetic elementopposite the third magnet and configured such that, during operation ofthe device, an electric current passing with a first polarity throughthe write element tends to move the domain wall away from the firstmagnet and toward the second magnet, and an electric current passingwith a second polarity, opposite the first polarity, through the writeelement tends to move the domain wall away from the second magnet andtoward the first magnet.
 4. The device of claim 3 wherein the writeelement is positioned on the substrate between the substrate and thefree magnetic element, and the free magnetic element is positioned, atleast in part, between the write element and the third magnet.
 5. (Thedevice of claim 1 wherein the first and second magnets are made of acompound including cobalt, iron, and boron.
 6. The device of claim 1wherein the first magnet has a coercivity of between 500 Oe and 100 Oe,and the second magnet has a coercivity of between 1500 Oe and 3000 Oe.7. The device of claim 1 wherein the first and second magnets are madeof different magnetic materials.
 8. The device of claim 1 wherein thefree magnetic element, the first magnet, and the second magnet areelements of a memory cell.
 9. The device of claim 1, wherein the firstand second magnets are made of a same material, have a same thickness,and have aspect ratios that are different from one another.
 10. Thedevice of claim 1, wherein the first and second magnets are each on thefree magnetic element
 11. A device, comprising: a semiconductorsubstrate; a free magnetic element over the semiconductor substrate; afirst magnet on the free magnetic element, the first magnet having afirst value of coercivity; a second magnet on the free magnetic element,the second magnet having a second value of coercivity different from thefirst value of coercivity; and a heavy metal layer between the substrateand the free magnetic element.
 12. The device of claim 11, wherein thefirst and second magnets have respective polarities that areantiparallel with respect to each other.
 13. The device of claim 11,further comprising: a tunneling barrier layer on the free magneticelement; and a third magnet on the tunneling barrier layer.
 14. Thedevice of claim 13, wherein the third magnet includes a portion of afirst CoFeB layer and an electrode layer, the tunneling barrier layerincludes a portion of a MgO layer, and the free magnetic elementincludes a portion of a first CoFeB layer.
 15. The device of claim 13,wherein a the third magnet is positioned laterally between the firstmagnet and the second magnet.
 16. The device of claim 15, wherein thewrite element is positioned on the semiconductor substrate between thesemiconductor substrate and the free magnetic element, and the freemagnetic element is positioned, at least in part, between the writeelement and the third magnet.
 17. A device, comprising: a semiconductorsubstrate; a free magnetic element on the semiconductor substrate, thefree magnetic element having a domain wall and having a first end and asecond end; a tunneling barrier layer on the free magnet element; afirst pinned magnetic layer over the tunneling barrier layer; a secondpinned magnetic layer over the substrate adjacent the first end of thefree magnetic element; and a third pinned magnetic layer on thesemiconductor substrate adjacent the second end of the free magneticelement; wherein the second and third pinned magnetic layer have a samematerial and a same thickness, and have aspect ratios that are differentfrom each other.
 18. The device of claim 17, wherein the second pinnedmagnetic layer has a polarity that is antiparallel relative to apolarity of the third pinned magnetic layer.
 19. The device of claim 17,wherein the second pinned magnetic layer has a coercivity that isdifferent from a coercivity of the third pinned magnetic layer.
 20. Thedevice of claim 17, wherein at least one of the first pinned magneticlayer and the second pinned magnetic layer is over the free magneticelement.